The present disclosure generally relates to rheological properties, and, more specifically, to methods for determining the rheology of drilling fluids in a wellbore.
Drilling fluids, also known as drilling muds, are specially designed treatment fluids that are circulated through a wellbore to facilitate a drilling operation. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. Drilling fluids can be oil-based or water-based, and the choice of a particular type of drilling fluid may be influenced by various factors. Functions of a drilling fluid during a drilling operation can include, for example, removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in the support of the drill pipe and the drill bit, and maintaining sufficient wellbore pressure to provide wellbore integrity and to prevent blowouts from occurring.
Although the hydrostatic pressure provided by drilling fluids is desirable to stabilize the subterranean formation and to contain fluids in the wellbore, formation damage and fluid loss can result if the wellbore pressure is excessive. Correspondingly, if the wellbore pressure is too low, formation fluids may enter the wellbore and create a blow out situation. Accordingly, it can be desirable to know the equivalent circulating density (ECD) of a drilling fluid in a wellbore in order to help maintain the wellbore pressure in a desired range. ECD represents the combined effect of hydrostatic fluid pressure, hydraulic pressure losses and choke pressure, among other factors. Hydraulic models can be used to predict the ECD and manage wellbore pressures during a drilling process. By applying hydraulic models, a well operator can better regulate and optimize a drilling operation by effectively managing wellbore pressures and maximizing the rate of penetration of the drill bit into the subterranean formation. Such modeling results can be compared to pressure-while-drilling (PWD) measurements in order to actively manage a drilling process by regulating factors such as, for example, pump rates, drill bit rotation rates, rates of penetration, choke pressures, and tripping speeds, not to mention varying the composition of the drilling fluid itself. In the absence of direct pressure measurements, such as PWD, reliance on hydraulic models may be especially important.
The rheology of a drilling fluid can determine whether it is able to deliver sufficient cuttings transport and sag resistance while maintaining pressure in a wellbore under a particular set of temperature and pressure conditions. Effects of inadequate rheological properties in a drilling fluid can include, for example, pressure loss in the wellbore, blowouts, weighting agent sag, poor cuttings transport, stuck pipe and the like. Excessive hydrostatic pressures resulting from inadequate rheological properties can also lead to issues such as lost circulation and unintentional fracturing.
Temperature and pressure can significantly impact the rheology of a drilling fluid. Although a drilling fluid may have an initial rheological performance resulting from its formulated composition, the rheological performance can change due to added materials (e.g., drill cuttings) entering the drilling fluid in-process during a drilling operation. The continual influx of drill cuttings and other added materials to a drilling fluid during a drilling operation in a wellbore can significantly complicate the determination of the drilling fluid's rheological performance. Drill cuttings and other added materials from the wellbore may be highly variable in nature, and the amount and identity of added materials present in the drilling fluid at any given point in time can fluctuate. Such variability can make it difficult to determine true composition of a drilling fluid and its associated rheological properties at any given time or wellbore locale, particularly when considering the further factors of temperature and pressure variance within the wellbore. Accordingly, it is often difficult to accurately model the downhole rheological performance of a drilling fluid based only upon measurements obtained in a laboratory setting.
Certain rheological properties, such as a fluid's change in apparent viscosity as a function of temperature and/or pressure, may be readily measured over a wide range of temperature and pressure conditions. However, some rheological properties can be difficult to determine under extreme temperature and pressure conditions, even in a laboratory setting, due to instrumental limitations. For example, shear stress and shear rate are rheological quantities that may be especially useful in determining a drilling fluid's ECD in a wellbore, but they can be difficult to measure under extreme temperature and pressure conditions. Although shear stress and shear rate may be readily measured in a laboratory setting at routine temperatures and pressures using a couette-style viscometer (e.g., a Fann 35 viscometer), many couette-style viscometers are completely unsuitable for use in the extreme temperatures and pressures that are commonly encountered downhole. Those that are adaptable to extreme temperature and pressure conditions (e.g., Fann 75 and Fann 77 viscometers) are exceedingly cumbersome and time-consuming to use. Accordingly, there is presently no simple way to determine certain high-interest rheological quantities of a drilling fluid under the temperature, pressure and compositional in-process conditions present within a wellbore. Other factors that may be of note in determining such rheological quantities in a wellbore environment include the variable and transient temperatures of a fluid progressing through the wellbore due to heat transfer to and from the subterranean formation, and extreme swings in temperature range, such as those encountered in deepwater drilling operations.